Diffusion-weighted imaging (DWI) techniques, including diffusion tensor imaging (DTI), are now among the most powerful MR imaging tools for assessing the neuronal microstructures in vivo [1, 2, 3]. To date, DWI data are commonly acquired with single-shot pulse sequences, such as single-shot echo-planar imaging (EPI) [4] and single-shot spiral imaging, to avoid significant artifacts resulting from amplified motion-induced phase errors in segmented DWI [5]. However, the image quality can be low and the spatial resolution limited in single-shot DWI [6]. The significant geometric distortions and limited spatial resolution make it difficult to measure diffusion properties at high spatial fidelity [7].
Recent efforts have been made to address the limitations of single-shot DWI. First, using parallel imaging techniques (e.g., SMASH [8], SENSE [9] and GRAPPA [10] among others), EPI and spiral imaging based DWI with reduced geometric distortion can be reconstructed from under-sampled k-space data at a chosen acceleration factor. A major concern with the conventional parallel MRI procedures is that the noises may be undesirably amplified, especially when a high acceleration factor is used. Second, segmented EPI, spiral imaging and fast spin-echo pulse sequences with embedded navigator echoes have been developed to produce DWI data that are less distorted and insensitive to motion-induced phase variations between shots, with shortened readout window and phase correction based on navigator signals [11, 12, 13, 14, 15, 16]. It has also been shown that the linear terms of motion-induced phase errors may even be estimated from segmented DWI with an iterative computation algorithm in post-processing without exclusively relying on navigator echoes [17, 18]. A clear advantage of the navigator-based segmented DWI, as compared with parallel DWI that is susceptible to undesirable noise amplification, is the high SNR. However, a potential concern for this technique is that the shot-to-shot phase variations due to local and nonlinear motion (e.g., in the brainstem [19]) can only be accurately measured with high-resolution navigator echoes, at the significant cost of imaging throughput.